Refactor exog_ts feature

I propose idea that we can pass exog time series directly in all nodes instead manually creating complicated pipelines with different time-series sources.

Now we need to create input data for all exogenous time series and pass them as independent node. I think it's not user-friendly behaviour.

I propose we should create a field in input data named "exog_ts" where we can store exogenous time series. It seems no problem because in time series forecasting problem we use only 2 types of nodes could handle exogenous variables.

* lagged - we can simply convert all ts to tables and concatenate them.
* time series models like ARIMA - we always pass raw (or processed) time series to them and we can pass exog_ts to them too.

Ideally it should look like this

Any advises are welcomed.